materials that interfere with RF

Discussion in 'Rocketry Electronics and Software' started by JohnCoker, Jan 13, 2020.

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  1. Jan 13, 2020 #1

    JohnCoker

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    OK, so what is it exactly that attenuates RF? Let's say I want an electronics bay with a threaded rod spine, but don't want that spine to attenuate the signal from a parallel antenna.

    Presumably steel and aluminum attenuate RF because of their conductivity. Presumably Nylon would not for the same reason. How about titanium (which has 3% the conductivity of copper)?
    https://www.mcmaster.com/catalog/126/3366
     
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  2. Jan 13, 2020 #2

    jderimig

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    The occasional threaded rod in the av-bay doesn't attentuate RF. At worse it may detune the transmitting antenna a little if its very close and the right length.
     
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  3. Jan 13, 2020 #3

    John Kemker

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    Carbon fiber will attenuate RF. I would not consider putting a transmitter inside a carbon fiber airframe/nosecone without routing the antenna outside.

    In addition, certain fillers/pigments/etc in fiberglass can attenuate RF. Think about the zinc used to give epoxy some UV protection, or the metal flakes in paint/gel coat.

    As John D. mentioned, the threaded rod wouldn't necessarily attenuate, but instead either detune, giving you a less-than-optimal match to your antenna, or cause directivity issues, giving you more gain in one direction and less in another. The classic Yagi-Uda antenna is an application of this phenomenon in order to create a directive antenna system. It is most likely to cause problems if it's resonant at the frequency of the radio. So, if I were using a 70cm Eggfinder TRS and placed a threaded rod that is 1/4 wavelength of my frequency parallel to my antenna, I would expect issues. However, something that is either longer or shorter by a significant amount, I wouldn't worry as much about.
     
  4. Jan 13, 2020 #4

    g.pitts

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    That's the answer I selected for the electronics bay I'm designing for my L2 certification (includes a tracker).
     
  5. Jan 13, 2020 #5

    Voyager1

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    The main issues with the proximity of metal threaded rods to a typical monopole antenna, as pointed out above by the two Johns, are the effect it has on the antenna impedance and directivity. Changes in antenna impedance at the operational frequency will detune the antenna leading to poor SWR. Changes in directivity will distort the radiation pattern such that in some directions the antenna will be less efficient.

    To illustrate the effect on directivity, the two plots below show the simulated radiation patterns of a 1/4 wave monopole antenna (wire 1, in the top plot) in free space and also placed parallel to two 8" x 1/4" threaded metal rods (wires 2 and 3, in the bottom plot). Assume that the rods are below the lower surface of the avionics sled separated by 3", and the monopole antenna is above the upper surface of the sled towards the forward end, centered between the rods. The +Y axis is out from the top of the sled.
    monopole_no_rods_1.jpg monopole_with_rods_1.jpg.png
    Notice the distorted lobe in the 2nd plot. This is due to the proximity of the two rods (wires 2 and 3). Changing the relative position of the antenna wrt the rods will change the shape of the rear lobe (-Y axis).

    While I don't consider this to be a definitive description of the effects of metal rods parallel to an antenna, it does illustrate that you might have to think twice about the best location for an antenna. I will see if I can also perform some tests to determine how this issue effects the antenna impedance and resonant frequency.
     
    Last edited: Jan 13, 2020
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  6. Jan 13, 2020 #6

    Voyager1

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    After performing some simple tests with my NanoVNA (a small and cheap vector network analyser) with a short monopole antenna, it is apparent the the proximity of the threaded rods does have a significant effect on the antenna impedance and resonant frequency. Unfortunately, I didn't have a 900 MHz band monopole with the right connector, so I made these measurements with a 400 MHz band monopole.

    Displayed in the following images are: (1) The monopole antenna connected to the VNA; (2) The antenna with parallel Brass tubes, similar to the ones attached to the bottom of the avionics sled; (3) The antenna with parallel threaded Steel rods, typically used in avbays.

    In image 1 the VNA measures the resonant frequency of the monopole as approximately 388 MHz and the impedance approximately 49 Ohms after settling. In image 2 the resonant frequency is approximately 404 MHz and the impedance is approximately 58 Ohms. In image 3 the resonant frequency is also 404 MHz and the impedance is approximately 62 Ohms.

    While these results are only very rough, they do indicate the potential effects of the parallel metal objects on resonant frequency and impedance. These effects, and those in the previous post, are enough to cause me to stick with threaded Nylon rods in my avbays.

    mono_1.jpg mono_with_brass_tube_1.jpg mono_with_steel_rod_1.jpg
     
    Last edited: Jan 13, 2020
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  7. Jan 13, 2020 #7

    dhbarr

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    Thanks for this testing & analysis!
     
  8. Jan 13, 2020 #8

    timbucktoo

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    I’d be curious to see same test with the wire whip used on telemetrum or egg products.
     
  9. Jan 13, 2020 #9

    jderimig

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    Very nice measurements Voyager! The next step would be to determine what effect there is on radiated field strength. I suspect those would be harder to discern a difference.

    Those resonant frequency detunings are surprising to me. I would have expected more. What you have measured are very modest shifts. I suspect that on my of our popular RF products in rocketry that the antenna tune out of the box is much greater than what you measured.
     
  10. Jan 13, 2020 #10

    gtg738w

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    As noted above, things that are conductive don't really attenuate the signal but they are reflective. At worst case, a reflection perfectly out of phase with the signal could cancel it completely (like noise canceling headphones) but typically it just shows up as more in-band noise. Imagine having a conversation in a room filled with echos. The more noise there is, the harder it is to understand and turning up the transmit power just makes the echos louder too. The same is true for buildings or even the ground. That is why line of sight is so important for range.
     
  11. Jan 13, 2020 #11

    cerving

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    Wire whips are definitely not perfect, but they're easy to make and "good enough" for most purposes. For maximum performance, a properly tuned antenna is best... that's why we include the Linx antennas with the 70 cm Ham products. They cost more, but they have predictable SWR and gain. You can still mess them up by placing them next to allthreads, though... as Voyager1 found, the farther away you keep them from metal, the better.
     
  12. Jan 13, 2020 #12

    caveduck

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    Thanks Voyager1 for some nice data!! One more thing about orientation - here is the pattern for a pretty nice commercial monopole at 900 MHz:
    upload_2020-1-13_11-39-27.png
    In this diagram, the radial angles are the distance away from straight out through the antenna axis. Notice that nearly all the pattern goes in the direction the monopole is pointing (this varies by how the ground plane is set up). This means you really, really want the antenna pointing *down* after apogee deployment. I've seen flights where the GPS telemetry disappears after ejection but the rocket is recovered intact with the TX still operating; in most such cases the antenna was pointing the wrong way when under drogue.
     
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  13. Jan 13, 2020 #13

    OverTheTop

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    Hey Caveduck. Would you please post a link to that antenna? I have never seen a monopole with radiation in the direction the element is pointing. Thanks.
     
  14. Jan 13, 2020 #14

    John Kemker

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    I think that's a horizontal projection, ie: as seen from the side. Most monopoles (verticals) have similar patterns.
     
  15. Jan 13, 2020 #15

    Voyager1

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    Thanks John. This is hardly a rigorous test, though! My intention was to simply illustrate that things do change when metal objects come in close proximity to an antenna.

    I agree that the radiated field strength would be more challenging to measure a significant difference. Unfortunately, in retirement, I now don’t have access to an anechoic chamber for far-field measurements.

    It would be good to repeat these measurements with a professional bench VNA and under more rigorous conditions. You can’t expect too much from a $50 instrument.
     
  16. Jan 13, 2020 #16

    jsdemar

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    The affect on the field pattern and the detuning + loss will depend upon the permittivity and reflectivity of the material. It will depend upon whether the material is floating or grounded.

    The interfering material will modify the pattern significantly if it's in the near field vs the far field. Everything within an electronics bay will be within the near field for the frequencies we use. Dielectric materials (plastics and paper) will act as a lens. Higher dielectric materials will attenuate more. Metals will become a co-radiator or a grounded element, significantly changing the radiated pattern when in the near field.

    Above a few hundred MHz, dielectrics become frequency dependent and complex. Teflon is better than Nylon or Fiberglass, for example.
     
  17. Jan 13, 2020 #17

    Voyager1

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    Yes, I totally agree with these points. In fact, as an example of parasitic elements becoming co-radiators, the lower plot I presented above does indicate RF current (the purple curves) in the two metal rods parallel to the monopole.
     
  18. Jan 13, 2020 #18

    jderimig

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    In one of my rockets I tape the wire whip antenna to a threaded rod in the nose cone. Never had a problem getting a good signal back. Still have the rocket, never lost it. YMMV.
     
  19. Jan 14, 2020 #19

    JohnCoker

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    Very interesting stuff! So we don't really know which materials are better; they're just affect things in different ways depending on the TX frequency and rod length. So I guess I don't feel as bad about using an Aluminum threaded rod in a bay with a TeleMega...
     
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  20. Jan 14, 2020 #20

    AeroAggie

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    Cris, which Linx model are you using on the ham side? I only have your 900MHz stuff.
     
  21. Jan 14, 2020 #21

    OverTheTop

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    The only way to know for sure is to model it with a field solver and infer performance from that. Ground testing will probably give you the confidence you need to fly.
     
  22. Jan 14, 2020 #22

    cerving

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    The 70 cm antennas are the ANT-433-CW-QW (shorter antenna for the TRS) and ANT-433-CW-HW (longer antenna for the LCD receiver). They're about $10 each... I don't make anything more off the Ham kits by including the antennas, it's more of the principle of the thing. If you are getting the Ham version for more performance, we should at least include some decent antennas.
     
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  23. Jan 23, 2020 at 6:57 PM #23

    ksaves2

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    This thread is great as I've been burned with attenuation issues. The major problem that I had in the past was with the metallic paints and GPS trackers in the 70 cm band. Launched two Rockets on the same day, lost 1 and barely found the other 1.
    The paint was rattle can Rustoleum metallic. I simply will not fly a metallic painted rocket with a tracker sealed up inside. I have a tendency to use nosecone mounted trackers or an ebay where the antenna is on a bulkhead that is exposed after the apogee charge blows. I do have a Wildman Jr. I fly with a wire antenna stented in the upper, main parachute bay that has non-metallic paint on it. 12 of 13 flights completely out of sight with recovery every time on a Beeline 70cm GPS tracker. I do not know how the 900 MHz trackers will respond to metallic paint nor do I want to risk a rocket to find out. My first misadventure would have been avoided by an adequate ground test which I obviously did not do. Kurt
     
  24. Jan 24, 2020 at 12:21 AM #24

    Jay Dub 4009

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    Anyone have experience with silver paint on the nose cone and having a feather weight tracker in the nose cone? I want to put a featherweight in the iris I am building but am concerned about it
     
  25. Jan 24, 2020 at 12:23 AM #25

    Jay Dub 4009

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    Anyone have experience with silver paint on the nose cone and having a feather weight tracker in the nose cone? I want to put a featherweight in the iris I am building but am concerned about it
     
  26. Jan 24, 2020 at 12:46 AM #26

    John Kemker

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    I'd recommend against it for the reasons stated above.
     
  27. Jan 24, 2020 at 12:53 AM #27

    Jay Dub 4009

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    Yes I think the best thing would be to find a light grey replacement for the silver
     
  28. Jan 24, 2020 at 1:44 AM #28

    OverTheTop

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    My Nike Apache had the NC painted with a metallic aluminium paint. Telemetry good to 37500' on that flight. That's with a TeleMega running 435MHz.

    Attenuation could be very paint and process dependent.
     
    Last edited: Jan 24, 2020 at 1:54 AM
  29. Jan 24, 2020 at 11:43 AM #29

    AeroAggie

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    The Createx 4300 series metallics use non-metallic things like mica for the reflective bits and should be* completely RF transparent. I say should because I haven't tested it, but my info comes directly from Createx Tech Support.

    Stay away from the Createx 4100 series - that uses actual aluminum flakes.
     
  30. Jan 24, 2020 at 7:22 PM #30

    ksaves2

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    Yeah, I've heard from a person who said they had decent results with a 900Mhz GPS tracker with a metallic painted rocket and the attenuation can very well be frequency dependent. I still won't personally risk it. On my 10k Wildman "stretch" rocket flight where I only received one position at altitude, an eagle eyed bystander caught sight of the rocket in the terminal stage of descent under a full main "way the heck out there". I couldn't see it due to cataracts that have since been fixed. Got the rocket back in 10 minutes. Just for kicks, I made the attempt at downloading the kml/position file off the 70cm Beeline GPS memory. Lo' and beholden, even though the Rf couldn't get out, the GPS signal was received fine! Had 6 to 10 satellite lock and at least got a souvenir track out of it. The Rustoleum metallic paint did not attenuate the high frequency incoming GPS signals. I didn't turn the tracker off until I got home and it showed me parked at the pizza joint where we went after the launch.

    I had my Wildman Jr rocket I commented above in #23 already entirely painted with Rustoleum Metallic Red paint when I had the "attenuation" misadventure. Thank heavens it is a small rocket because I stripped the upper main chute bay of the red by dipping in a tray of acetone. I reprepped the surface, re-primed and shot a non-metallic yellow paint. The Beeline resides in the upper part of the ebay and the Raven altimeter underneath it. A small hole is drilled in the front bulkhead to allow the antenna to project into the yellow painted upper bay. When the Beeline is mounted, I use some clay to "grout" the hole and use the cardboard tube that comes with AT motor igniters to stent the antenna wire so it doesn't get smushed by the packed main chute. Works every time and gases don't get into the ebay. The nosecone and the lower part of the rocket are still metallic red and are of no issue.

    I was debating what to do with the totally metallic painted Wildman "stretch". It's stretched as I built it out of longer tubes and it actually has the length to take a 54mm Loki M motor although I didn't intend that as the rocket was built long before that motor was available. I just liked the look of a long necked rocket. I used Cotronics 4525 high temp/high strength epoxy with 5 centering rings on the longer motor mount tube with liberal use of the epoxy. It was loafing to 10k with an L1400 motor and no doubt could handle much higher thrust. Only problem is nearby launch sites couldn't handle the flight profile with an M and I don't have the L3 yet.

    I was going to aft mount the Beeline GPS in the ebay with a screw on bulkhead antenna mount so at apogee the antenna would be out in "free" air. That is a workable scenario as I used a longer tube for the ebay so there is plenty of room.
    Remember the GPS signal was received well through the Rustoleum metallic paint. What I've decided to do is simply buy a new nosecone and construct a non-metallic painted nosecone tracker bay. I then can convert the ebay to two
    deployment devices. I was flying it with a single deployment device an ARTS 2. I decided on two new altimeters as I shudder to think of something this big coming in as a core sampling or people skewering device. Kurt
     

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